This invention relates to a device for changing the number of the needles at work in a circular knitting machine for stockings and like articles.
Such changing is known to become necessary as the knitting of the heel portion of a stocking article begins, when the number of the needles at work must be first decreased and then increased gradually again to form the heel pouch. The change is effected by means of the so-called needle raising and lowering pickers, which during the reciprocating movement of the needle cylinder(s) are active to respectively exclude from knitting one or more needles, by moving them to a raised track whereat they are not knitting, and return them back to the knitting track. There are commonly provided two needle raising pickers, which are respectively located on either sides of a yarn or thread feed, being each adapted for operating in one direction of reciprocation of the needle cylinder(s), and one needle lowering picker, which is configurated to operate in both directions of reciprocation. The pickers act directly on the needles in the single cylinder machines, and on the sliders in the double cylinder machines.
The problems connected with these pickers have several aspects. In particular, it should be considered that at each stroke of the reciprocating motion of the needle cylinder the last needle which has picked up the yarn during the knitting of the first half of the heel pouch must then be excluded by the picker at the start of the following stroke. This fact implies that said last needle will no more be knitting at the successive stroke, while the feed yarn previously caught by said last needle winds itself around the stem of the needle as this is raised to be excluded. It follows that the yarn is stretched, the particular rippled configuration of the needle contributing to said stretching, which can easily result in the breaking of the yarn, and attendant discarding of the knitted product. That phenomenon is specially frequent when a comparatively weak yarn is used, or when a rather tight loop is being knitted.
Another problem, which however, contrary to the former, only affects those double cylinder machines which are equipped with false sinkers associated with the sinkers, originates from the fact that the raising of the sliders to exclude the respective needles may be obstructed by the protruding false sinkers as the latter, for a reason whatever such as because worn out or damaged, fail to slide in their seats properly and have difficulty to re-enter. In this case, the sliders, which are provided at the top with a tab, collide therewith, in their upward movement as produced by the picker, against the respective false sinkers, thereby they cannot reach the correct height. Thus, the cylinder rotation brings the sliders to strike the stationary cams with their butts, thereby the butts break and the machine must be stopped to replace the damaged sliders, with obvious considerable losses in the production output.